During walking in footwear or bare feet ground reaction forces (GRF's) act on the sole of the foot. After the heel strikes the ground the GRF can rise to a maximum of 100% to 140% of a person's body weight. As the force increases to this maximum, usually, there is an oscillation in the magnitude of impact force, known as the "heel strike transient".
The impact force causes a mechanical shock wave known as "impact shock" to propagate through the skeletal system up to the skull. The energy of this shock wave is dissipated as it propagates through bone, soft tissue and muscle. The degree of dissipation can vary depending on the motion and muscle action at the joints, particularly the joints of the lower limbs, and any degenerative changes that may have occurred at the joints.
The heel pad is a fatty fibrous structure that, in a healthy state, is capable of absorbing up to 80% of the heel strike peak acceleration propagated to the tibia. The heel pad can have better shock absorbency than Sorbothane (Trade Mark) or EVA foam which are commonly used in good quality running shoes.
The effectiveness of the body's natural shock absorbing mechanisms can be reduced in the case of musculoskeletal disease, trauma or mechanical fatigue. Lack of adequate shock absorption can cause larger acceleration transients to propagate through the skeletal system. Larger impact forces can result in overuse injury and mechanical fatigue at the joints of the lower limbs and in the spine.
The shock absorption capabilities of the heel pad can be enhanced by wearing footwear that has a heel counter that confines the heel pad and by placing a shock absorbing material or device (such as an air system, liquid system, and valve system) under the heel to absorb impact energy generated at heel strike and thereby reduce the magnitude of the impact force. The effectiveness of known systems varies considerably. Moreover, in many instances, known systems require the addition of components to a conventional article of footwear and therefore increase the material costs and make more difficult the manufacture of the footwear.